These steels are, in principle, ferritic at all temperatures. This is achieved by a low content of austenite forming elements, mainly nickel, and a high content of ferrite forming elements, mainly chromium.
Ferritic types, such as 4003 and 4016, are mainly used for household utensils, catering equipment and other purposes where corrosion conditions are not particularly demanding.
Steels with high chromium content, such as 4762 with 24% chromium, are used at high temperatures where their resistance to sulphurous flue gages is an advantage. However, the risk of 475 °C embrittlement and precipitation of brittle sigma phase in high-chromium steels must always be taken into consideration.
Ferritic steels, such as 4521 with extremely low carbon and nitrogen contents, find greatest use where there is a risk of stress-corrosion cracking.
Ferritic steels have slightly higher yield strength (Rp 0.2) than austenitic steels, but they have less elongation at fracture. Another characteristic that distinguishes ferritic steel from austenitic material is that ferritic steels have much lower strain hardening.
High Temperature Ferritic Stainless Steels
Chromium - the main alloying element – produces a transformation free ferritic microstructure. Additions of aluminium and silicon stabilize the ferrite and increase scale resistance. Excellent resistance to oxidizing and reducing sulphurous gases, compatibility with molten metals, high thermal conductivity and low thermal expansion, are features that complement the properties of our existing high temperature austenitic stainless steels. Four grades with alternating chromium contents allow a broad variety of heat and corrosion resistance according to the requirements in the respective application. The two lower alloyed grades - 4713 and 4724 - are best suited for oxidizing sulphur environments and exposures to the “low” high temperature ranges, where austenitic stainless steels undergo structural changes that lead to embrittlement.
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4713 is best suited for service temperatures between 550 and 800°C. Owing to its low chromium content (6.5%) it may be exposed to moderately corrosive environments, but it will withstand oxidizing sulphur attacks. It is commonly used in the furnace and glass industries, whilst exotic utilisations of 4713 as frying and grill plates for the catering business prove its variety of individual applications. |
Application of 4713 as grill bar
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Application of 4713 as industrial fan
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4724 is also applicable in oxidising sulphur-containing atmospheres. The higher chromium content of 13% enhances its scale resistance up to 850°C. |
With higher chromium contents, 4742 and 4762 cover applications that require higher service temperatures and insensitivity to reducing sulphurous gases. Since austenitic stainless steels form low-melting-point nickel sulphides, which destroy the protective oxide layer, ferritic material is recommended. 4742 can be applied in the range 700 - 1000°C. With 18% chromium, it shows very good resistance to reducing sulphurous gases in addition to oxidizing sulphur-containing atmospheres.
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4762 is the highest alloyed grade with 24% chromium and is therefore best in terms of heat and sulphur resistance. Rotary drums or tubes in chemical facilities passing aggressive H2S or SO2, are typical applications for 4762. The power industry has utilised this grade for coal burners. Further, it can be used for components handling molten copper, where nickel contamination is to be avoided. Service temperatures range from 800 – 1150°C. |
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Application of 4762 as combustion chamber
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All four grades are included in the EN 10095 – the standard for heat resistant steels and nickel alloys. They are produced as quarto plates from 2 mm in thickness. Taylor made plates allow individual dimensioning, provide ready to use material, and thus eliminate scrap.
Datasheets on the high temperature ferritic stainless steels further inform about mechanical and physical, corrosion resistance and give recommendations for proper fabrication etc.